Oxymethylene polymers (also referred to as polyoxymethylenes and polyacetals) are a class of thermoplastics that have been found useful for a variety of applications including automotive construction, electrical applications, and medical technologies. Polyoxymethylenes have excellent properties including mechanical (e.g., strength) characteristics, permeability resistance, fatigue resistance, abrasion resistance, chemical resistance, and moldability.
Unfortunately, polyoxymethylenes tend to degrade when heated and are unstable, particularly in an oxidative, basic, or acidic environment. Moreover, as the polymers degrade they emit degradation products such as formaldehyde as volatile organic compounds (VOCs). Formaldehyde as well as other VOC emissions lead to an unpleasant odor and can raise environmental health concerns. The emission of VOCs and particularly the emission of formaldehyde by polyoxymethylenes both during processing and after the polymer has been molded into a desired shape have been problematic for the industry and has impaired the usefulness of these materials in many application sectors. For instance, the automotive industry has developed analysis methods (see, e.g., German Automotive Industry Recommendation No. 275, documented by Kraftfahrwesen e.V., July, 1994) for determining formaldehyde emission from polyoxymethylene components and strongly encourages the development of low emission polyoxymethylene polymers.
In an attempt to limit the VOC emissions of oxymethylene polymers, a variety of polymer stabilizers have been developed. For instance nitrogen-containing compounds such as dicyandiamide and amino-substituted triazine compounds have been utilized as polymer stabilizers. These stabilizers generally function as formaldehyde scavengers to bind formaldehyde and prevent emission. Unfortunately, such stabilizers do nothing to prevent the degradation of the polyoxymethylene polymers that leads to the formaldehyde emission in the first place. In addition, as stabilizers are not bound to the oxymethylene polymer, they have a tendency to migrate to the surface of the formed products and cause deposits and surface imperfections on the products.
What is needed in the art are polyoxymethylenes that are resistant to degradation, so as to provide polymers, polymer compositions, and products formed from the polymers that exhibit low VOC emission, have little or no mold deposits and improved surface characterization, present desirable strength and resistance characteristics, and are suitable for use in a variety of applications.